Enhanced Reactivities of Iron(IV)‐Oxo Porphyrin π‐Cation Radicals in Oxygenation Reactions by Electron‐Donating Axial Ligands

Abstract

The proximal axial ligand in heme iron enzymes plays an important role in tuning the reactivities of iron(IV)-oxo porphyrin pi-cation radicals in oxidation reactions. The present study reports the effects of axial ligands in olefin epoxidation, aromatic hydroxylation, alcohol oxidation, and alkane hydroxylation, by [(tmp)(+*) Fe(IV)(O)(p-Y-PyO)](+) (1-Y) (tmp = meso-tetramesitylporphyrin, p-Y-PyO = para-substituted pyridine N-oxides, and Y = OCH(3), CH(3), H, Cl). In all of the oxidation reactions, the reactivities of 1-Y are found to follow the order 1-OCH(3) > 1-CH(3) > 1-H > 1-Cl; negative Hammett rho values of -1.4 to -2.7 were obtained by plotting the reaction rates against the sigma(p) values of the substituents of p-Y-PyO. These results, as well as previous ones on the effect of anionic nucleophiles, show that iron(IV)-oxo porphyrin pi-cation radicals bearing electron-donating axial ligands are more reactive in oxo-transfer and hydrogen-atom abstraction reactions. These results are counterintuitive since iron(IV)-oxo porphyrin pi-cation radicals are electrophilic species. Theoretical calculations of anionic and neutral ligands reproduced the counterintuitive experimental findings and elucidated the root cause of the axial ligand effects. Thus, in the case of anionic ligands, as the ligand becomes a better electron donor, it strengthens the FeO-H bond and thereby enhances its H-abstraction activity. In addition, it weakens the Fe=O bond and encourages oxo-transfer reactivity. Both are Bell-Evans-Polanyi effects, however, in a series of neutral ligands like p-Y-PyO, there is a relatively weak trend that appears to originate in two-state reactivity (TSR). This combination of experiment and theory enabled us to elucidate the factors that control the reactivity patterns of iron(IV)-oxo porphyrin pi-cation radicals in oxidation reactions and to resolve an enigmatic and fundamental problem.

Molecular Iron-Based Oxidants and Their Stoichiometric Reactions

“…An explanation for the rate acceleration could therefore simply be an enhanced stabilization of the high-valent species. The same trend is also observed for iron porphyrins [111] as covered in the next section.…”

“…Generally, it has been observed that oxidative power increases when the porphyrinic scaffold is decorated with electron-withdrawing substituents [117,118]. Somewhat counterintuitively, it has also been shown that the reactivity of [Fe IV O(TMP)( p-OPyY)] + complexes towards the electrophilic epoxidation of olefins and hydroxylation of aromatic and aliphatic substrates increases, when Y is an electron-donating group [111]. DFT calculations indicate that a complicated transeffect is at play.…”

The biology and chemistry of high-valent iron–oxo and iron–nitrido complexes

“…The axial ligands bound trans to the iron-oxo moiety also markedly influence the reactivities of oxoiron(IV) porphyrin π-cation radicals. For example, a recent study by Kang et al 30 (Fig. 5a) and theoretical methods, showed that rates of both the O-transfer and H-atom abstraction reactions of the porphyrin Figure 2 | Reactions catalysed by high-valent oxo intermediates of iron-containing enzymes.…”

Catalytic Epoxidation of Alkenes by the Manganese Complex of a Reduced Porphyrinogen Macrocycle

“…[4a,8c, 13,24] In particular, it was shown that N-donor ligands such as imidazole derivatives play a key role in Mn-porphyrin-or Mn-salen-catalyzed olefin epoxidations by H 2 O 2 . We tested the influence of a variety of additives on the styrene epoxidation catalyzed by complex 2 with H 2 O 2 in acetonitrile, by comparing the conversion into styrene oxide obtained after 90 min in the presence and in the absence of the additive.…”